1. Field of the Invention
The present invention relates to a multi-function product, and more particularly, to an apparatus for protecting a multi-function product connected to a network to perform an appropriate function from a surge voltage in the public telephone network, and sectionalizing the multi-function product into an idle state and a loop state.
2. Description of the Related Art
Multi-function products (MFP) such as office automation instruments can perform a number of functions such as a telephone function, a printer function, a copier function, a scanner function, and a fax function.
FIG. 1 is a block diagram showing a configuration of a conventional apparatus for protecting an MFP from a surge voltage. The apparatus for protecting the MFP from the surge voltage can also be applied to general digital copiers and scanners. Here, for example, an MFP is described.
Referring to FIG. 1, the MFP includes a main portion 100, a network portion 200, a data transmission unit 300 transmitting data between the main portion 100 and the network portion 200, a public telephone network connection unit 400, and an external apparatus connection unit 500.
The main portion 100 generally includes a central processing unit (CPU) 110, a memory 120, a modem 130, and a network interface unit 140.
The network portion 200 includes a network portion processing device (IC) 210 which is equipped with a modem interface unit 211, an A/D-D/A conversion unit 212, a level conversion unit 213, a ring signal detection unit 214, an alternating current signal input unit 215, a direct current control unit 216, and an alternating current signal output unit 217, a bridge diode BD, capacitors C1, C2, and C3, resistors R1 and R2, transistors Tr1 and Tr2, a surge voltage cut-off device V1, and a switching unit SW1.
The data transmission unit 300 is interposed between the main portion 100 and the network portion 200, and the data transmission unit 300 transfers arranged data signals and clock signals.
The main portion 100 processes fax signals and aural signals. The modem unit 130 is connected with the data transmission unit 300 via the network interface unit 140 and sends/receives fax signals and aural signals.
The bridge diode BD of the network portion 200 always applies a forward voltage to the transistors Tr1 and Tr2 even if the polarity of a switching system connected to the public telephone network connection unit 400 is reversed. The transistor Tr1, the resistor R1, and the direct current control unit 216 allow a predetermined direct current to flow into the public telephone network connection unit 400. The transistor Tr2 and the resistor R2 amplify alternating current signals output from the alternating current signal output unit 217 and send them to the public telephone network connection unit 400. The capacitor C1 removes a direct current component from alternating current signals input via the public telephone network connection unit 400 and delivers them to the alternating current signal input unit 215. The capacitors C1 and C2 deliver ring signals input from the public telephone network connection unit 400 to the signal detection unit 214. The surge voltage cut off device V1 protects the network portion process device (IC) 210 from a surge voltage input from the public telephone network connection unit 400. Generally, a varistor or a gas tube is used as the surge voltage cut off device V1. The switching unit SW1 switches to connect an external apparatus and a multi-function product via the external apparatus connection unit 500 when an additional external apparatus is connected to the multi-function product.
In a multi-function product using the network portion process device (IC) 210, the surge voltage cut off device V1 is arranged adjacent to the public telephone network connection unit 400 in order to protect the network portion process device (IC) 210 from the surge voltage input from the public telephone network connection unit 400. In a multi-function product using the network portion process device (IC) 210, when the surge voltage cut off device V1 is connected between a ring signal path and a loop current path, a device having an operating voltage greater than the peak value of the ring signal is used as the surge voltage cut off device V1. For example, since a peak value of the ring voltage is 212V when the ring voltage is 150V, the surge voltage cut off device V1 must be used at least 212V.
In an idle state (a current path like a dotted line of FIG. 1) in which a voice or a fax signal is not sent/received in the multi-function product, since transistors Tr1 and Tr2 are all off, the multi-function product becomes in a high impedance state and a surge voltage current input from the network connection unit 400 becomes insignificant. Therefore, the line portion process device (IC) 210 can be protected by the surge voltage cut off device V1.
However, in a loop state (a current path like a solid line of FIG. 1) in which a voice or a fax signal is sent/received via in the multi-function product, since a terminal enters a low impedance state, surge voltage, that is, a high current instantly, flows from the network connection unit 400, and thus the network portion process device (IC) 210 can be damaged. To prevent this, an input path and a grounding path of the line portion process device (IC) 210 have to be sufficiently isolated from a path into which surge voltage is input. However, to make a design as described above, there are many restrictions in component arrangement or layout.